Magnetic recording media and magnetic storage device

ABSTRACT

It is an object of the present invention to provide a magnetic recording medium which has a high level of magnetic anisotropy energy, retains stably the state of recording magnetization within the operating temperature range and is suitable for high-density recording with a low noise level, and a magnetic storage device for the magnetic recording medium. For attaining the above object, the magnetic recording film of the magnetic recording medium is characterized in that its magnetic anisotropy energy constant K u  at an absolute temperature of 300 K is greater than 3.6×10 6  erg/cc and the average particle diameter is larger than 5 nm and smaller than 12 nm.

BACKGROUND OF THE INVENTION

The present invention relates to a magnetic recording medium suitablefor high-density recording and also to a magnetic storage device withsaid magnetic recording medium.

The magnetic recording system is divided into two types, that is,longitudinal recording and perpendicular recording, with the formerbeing widely prevalent. The longitudinal magnetic recording systemcarries out magnetic recording by forming recording bits by the magneticfield generated by the magnetic head in such a way that the N-pole ofone bit butts against the N-pole of its adjacent bit and the S-pole ofone bit butts against the S-pole of its adjacent bit, the recording bitsbeing arranged parallel to the plane of the magnetic recording medium.For this recording system to have a high recording density and togenerate a high reproducing output, it is essential to reduce the effectof demagnetizing field on the recorded bits. To this end, attempts arebeing made to reduce the thickness of the magnetic film and to increasethe coercive force in the magnetic film.

The perpendicular recording system performs magnetic recording in thefollowing way. Recording bits are formed by the magnetic field of themagnetic head in the direction perpendicular to the film plane of themagnetic recording medium having the perpendicular magnetizinganisotropy, with adjacent bits being magnetized in the anti-paralleldirection. Thus, the magnetic pole of one bit has a polarity opposite tothat of its adjacent bit. As a result, the magnetic moments of adjacentbits attract each other. This stabilizes magnetization for recording andincreases the coercive force, thereby contributing to high-densityrecording.

In both recording systems, an increase in coercive force is an importantfactor to improve the recording density. One of the factors to determinecoercive force is magnetocrystalline anisotropy energy. This is ameasure to indicate the ease with which the magnetic moment in magneticcrystal grains is oriented in a specific crystalline direction. Thegreater the value, the easier the orientation. For example, in the caseof Co crystal grains, the magnetic moment easily orients in thedirection of the c axis of the hexagonal closed-pack crystal lattice.(This is the axis of easy magnetization.) The magnetocrystallineanisotropy energy (or the magnetic anisotropy constant) K_(u) is 4.6×10⁶erg/cm³.

The energy to orient the magnetic moment in crystal grain in thedirection of axis of easy magnetization is given by K_(u)V, where V isthe volume of crystal grain. On the other hand, the magnetic momentfluctuates due to thermal vibration. The energy of thermal vibration isgiven by k_(B)T, where k_(B) is Boltzmann constant and T is an absolutetemperature.

The behavior of the magnetic moment varies depending on the relativemagnitude of k_(B)T and K_(u)V. If k_(B)T<<K_(u)V, the magneticanisotropy energy is sufficiently large and hence the magnetic momentorients approximately in the direction of the c axis of crystal grain.If k_(B)T>>K_(u)V, the energy of thermal vibration is larger than themagnetic anisotropy energy and hence the magnetic moment continuesthermal vibration (super paramagnetic state). This thermal vibrationcauses the inversion of magnetic moment to take place with a certainprobability per unit time. For example, the energy of thermal vibrationrequired for the inversion of magnetic moment to take place with aprobability of 1/e per second is 25 k_(B)T. If this inversion takesplace, the coercive force decreases as time lapses along with theprobability, resulting in a decrease in recording density. Therefore,the recording medium should at least meet the condition of 25k_(B)T<<K_(u)V.

In the meantime, among the related art media for high-density magneticrecording is magnetic film of Co₈₁Cr₁₅Ta₄ alloy. (See IEEE Transactionof Magnetics, vol. 34, No. 4 (July 1998), pp. 1558-1560, as the first USliterature.) This magnetic recording medium has a magnetic anisotropyenergy K_(u) of 1.3×10⁶ erg per cm³ at about 300 K (absolute temperatureT).

The above-mentioned medium is characterized by a magnetic grain size ofabout 15 nm (on average) and a film thickness of about 20 nm. Themagnetic anisotropy energy possessed by a single magnetic crystal grainis K_(u)V=4.6×10⁻¹² erg. On the other hand, the energy of thermalvibration at room temperature (300 K) is k_(B)T=4.1×10⁻¹⁴ erg. Thus,K_(u)V>>25 k_(B)T. In other words, under the present condition ofcrystal grain size, the magnetic anisotropy energy is much larger thanthe energy of thermal vibration, and hence the magnetic moment is fixedin the direction of axis of easy magnetization and this leads to asufficiently large coercive force.

For both recording systems to have an increased recording density, it isimportant not only to increase the reproduction output for high-densityrecording but also to reduce the noise of the medium. The noise of themedium in a state of high-density recording results from the zigzagmagnetic domain wall in the transition region of the recording bit. Thegreater the fluctuation of the magnetic domain wall, the greater thenoise. Thus, common practice to decrease noise is to reduce the particlesize of the magnetic crystal grains constituting the magnetic recordingmedium, thereby to reduce the fluctuation of the magnetic domain wall inthe transition region.

The related art recording density (as experimental data) is 10 Gbit persquare inch (as stated at the 7^(th) MMM-Intermag Joint Conference(January 1998, Session ZA). This recording density corresponds to alinear recording density of about 400 kFCI (Flux Change per Inch, ormagnetization reversal number per inch), with the bit length being about60 nm, assuming that the ratio of bit length to track width is about20:1, which is common.

SUMMARY OF THE INVENTION

The thin-film medium for longitudinal magnetic recording now in use hasa crystal grain diameter of about 15 nm. This implies that only fourcrystal grains arranged in the bit direction currently constitute a bitwhich is 60 nm long. This results in a large zigzag magnetic domain wallin the transition region. In other words, the magnetic domain wallfluctuates so greatly as to give rise to a problem in noise.

If the related art medium described in the first literature given aboveis designed such that the crystal grain size is 10 nm and the filmthickness is 10 nm according to the related art technique so as to raiserecording density and reduce noise, then the magnetic anisotropy energyof the crystal grains will be K_(u)V=1.2×10⁻¹² erg. This value is aboutone-forth of that possessed by the related art magnetic crystal grainbefore design. However, the relation of K_(u)V>25 k_(B)T is satisfied.

Unfortunately, simply reducing the crystal grain size and film thicknessas mentioned above results in a medium which has a low coercive force atthe higher operating temperature range (as mentioned later), and thismedium does not produce sufficient high reproducing output. In otherwords, as the temperature of crystal grains rises by 50 K, reaching 350K, the energy of thermal vibration increases to k_(B)T=4.8×10⁻¹⁴ erg. Onthe other hand, the magnetic anisotropy energy usually decreases withincreasing temperature, and it disappears at the Curie point. In thecase of Co₈₁Cr₁₅Ta₄ alloy described in the first US literature mentionedabove, K_(u)1.3×10⁶ erg/cc at T=300 K, while K_(u)=1.0×10⁶ erg/cc atT=350 K. In other words, a temperature rise of 50 K causes K_(u) todecrease by 20% or more.

Consequently, the magnetic anisotropy energy of a single crystal grainat T=350 K is K_(u)V=7.9×10⁻¹³ erg, and the relationship between theenergy of thermal vibration and the magnetic anisotropy energy becomesK_(u)V<25 k_(B)T. The result is that the magnetic moment in a crystalgrain is hardly fixed in the direction of axis of easy magnetization,and this in turn leads to a decreased coercive force and an unstablestate of recording magnetization.

The present invention contributes to eliminate the above-mentioneddisadvantages involved in the related art technology. It is an object ofthe present invention to provide a magnetic recording medium which has ahigh level of magnetic anisotropy energy, retains stably the state ofrecording magnetization within the operating temperature range, and issuitable for high-density recording with a low noise level. It isanother object of the present invention to provide a magnetic storagedevice with said magnetic recording medium.

The above-mentioned object of the present invention is achieved by amagnetic recording medium which is characterized in that the magneticanisotropy energy at 300 K is K_(u)≧3.6×10⁶ erg/cc and the averageparticle diameter (d) of magnetic crystal grains is 5 nm<d<12 nm. (“d”is defined as the diameter of a circle having the same area as that of amagnetic crystal grain in the direction of the plane of the film.) Themagnetic recording medium like this can be realized by the steps offorming an underlying film for orientation control, forming thereon anunderlying film for lattice alignment, forming thereon a film of Co—Cralloy magnetic body containing an added element, and performing heattreatment, thereby diffusing the added element into the crystal grainboundary. (This process will be explained later in more detail.)

The diameter of magnetic crystal grain was established as mentionedabove for the following-reason. As the grain size increases, thefluctuation of the zigzag magnetic domain wall in the transition regionincreases due to the crystal size distribution and the crystal grainarrangement distribution, thus increasing medium noise resulting fromtransition noise. By contrast, as the grain size decreases, the volumeof crystal grain decreases and hence the magnetic anisotropy energydecreases. The optimum range was established in consideration of thesetwo points.

In the case where the linear recording density is 400 kFCI (as mentionedabove) and the bit length is about 60 nm, the number of crystal grainsconstituting the bit in the lengthwise direction should be at least 5.Four crystal grains are insufficient to eliminate noise. (This has beenfound from the study of noise.) The foregoing leads one to conclude thatthe average particle diameter of magnetic crystal grains constitutingthe magnetic recording film should be smaller than 12 nm. On the otherhand, if the particle diameter of magnetic crystal grains is 5 nm orsmaller, the volume of each crystal grain is excessively small. Theresult is that the energy of thermal vibration of magnetic moment islarger than the magnetic anisotropy energy of magnetic crystal grains tokeep their magnetic moment in the direction of axis of easymagnetization. Thus, the magnetic moment cannot stably orient in thedirection of axis of easy magnetization, and the magnetic crystal grainsexhibit the properties of super paramagnetism. For this reason, theaverage particle diameter (d) of magnetic crystal grains should bewithin the range of 5 nm<d<12 nm.

The above-mentioned magnetic anisotropy energy K_(u) was derived fromthe following view point. For the state of recording magnetization toremain stable regardless of temperature change, it is necessary that theratio of K_(u)V/k_(B)T (where K_(u)V is the magnetic anisotropy energyof magnetic crystal grains and k_(B)T is the energy of thermal vibrationof magnetic crystal grains) should have a sufficiently large valuewithin the operating temperature range of the magnetic storage device orthe magnetic recording/reproducing apparatus.

The relation between the magnetic moment energy K_(u)V and the energy ofthermal vibration k_(B)T was studied from the view point of thestability of reproducing output with time. (IEEE Transaction ofMagnetics, vol. 33, No. 5 (September 1997), pp. 3028-3030, as the thirdUS literature.) The authors of the literature measured the output ofreproducing signals from the head immediately after bit recording andafter standing for 96 hours. They found that the output decreased byonly 4% after standing 96 hours if the k_(u)V/k_(B)T is about 85,whereas the output decreased by 10% or more after standing 96 hours ifthe K_(u)V/k_(B)T is about 55.

A decrease in output by 4% after standing 96 hours is desirable for therecording/reproducing. characteristics of the magnetic storage device.Therefore, for the recording magnetization to remain stable in theoperating temperature range of the magnetic storage device, it isnecessary that the condition of K_(u)V/k_(B)T>85 should be satisfied inthe neighborhood of T=350 K which is the upper limit of the operatingtemperature. Also, in order to realize the high-density recording, it isnecessary that the average particle diameter of crystal grains should bein the range of 5 nm<d<12 nm. It follows from this crystal grain sizethat the magnetic anisotropy energy of magnetic crystal grains whichmeets the condition of K_(u)V/k_(B)T>85 at T=350 K should be K_(u)(T=350 K)≧3.0×10⁶ erg/cc.

The magnetic anisotropy energy is the main cause of the coercive forceof the recording medium. Therefore, when the magnetic anisotropy greatlyvaries within the operating temperature range of the magnetic storagedevice, the temperature dependence of coercive force also increasesaccordingly. Consequently, it is desirable to keep at about 10% thechange of coercive force within the temperature range from T=300 K toT=350 K. It follows from this that the change with temperature inmagnetic anisotropic energy should be such that K_(u)(T=350K)/K_(u)(T=300 K)≧0.85. This leads one to the conclusion that themagnetic anisotropic energy at T=300 K should be such that K_(u)(300K)≧3.6×10⁶ erg/cc and [K_(u)V/k_(B)T] (T=350 K)/[K_(u)V/k_(B)T] (T=300K)≧0.73.

As mentioned above, the magnetic recording medium for high-densityrecording should be able to keep the state of microcrystalline grainsand to stably retain the state of recording magnetization. To this end,it is necessary to make the magnetic recording film (layer) from amagnetic material which has a greater magnetic anisotropy energy thanthe related art one, and it is also necessary to minimize the changewith temperature in magnetic anisotropy energy within the operatingtemperature range of the magnetic storage device.

Incidentally, in order to reduce noise resulting from the zigzagmagnetic domain wall in the magnetization transition region of themedium, it is desirable to reduce the length and width of themagnetizing transition region between one recording bit and its adjacentbit.

The length and width of the magnetizing transition region of a medium isusually proportional to the product of “t” (the thickness of themagnetic recording film constituting the medium) and “Br” (the residualmagnetic flux density of the magnetic recording film of the medium).Therefore, the smaller the value of Br·t, the lower the noise level andthe better the S/N of the medium for high-density recording. On theother hand, a decreased value of Br·t leads to a decrease in magneticflux leaking from the recording bit, which reduces the output of thereproducing head.

For this reason, the value of Br·t should be in the range of 30Gauss·μm<Br·t<80 Gauss·μm, so as to keep the medium S/N high and toprevent the decrease of output in the case of high-density recording.

A mention is made below of the material from which the recording mediumis made. The magnetic anisotropy energy of a magnetic alloy is greatlyaffected by the combination and composition of elements constituting themagnetic material. In the case of an ordered alloy, it is greatlyaffected by whether the ordered alloy is in the ordered state ornon-ordered state. In general, if Co is incorporated with a noble metal(such as Pt) or a rare earth metal element (such as Sm), the resultingalloy increases in magnetic anisotropy. If Co is incorporated with anon-magnetic element (such as Cr), the resulting alloy decreases insaturation magnetization and also magnetic anisotropy accordingly. Asthe amount of non-magnetic element added increases, the curie pointdecreases, with the result that the magnetic anisotropy energy changesmore with temperature in the neighborhood of room temperature.

The magnetic recording medium is a polycrystalline thin film composed ofmagnetic crystal grains. The magnetic properties of each crystal grainare responsible for the macroscopic magnetic characteristics of the thinfilm as a whole. In other words, if individual magnetic crystal grainshave a large magnetic anisotropy energy, have a Curie point which issufficiently higher compared with the operating temperature range of themagnetic storage device, and changes less with temperature in magneticanisotropy energy in the operating temperature range, then the mediumfilm would have macroscopic magnetic characteristics similar to them.

Thus, it is a good practice to increase the ratio of ferromagneticelement, noble metal, and rare earth element in individual magneticcrystal grains and to decrease the ratio of non-magnetic added elementin them. However, simply increasing the ratio of magnetic elements inthe magnetic crystal grains causes exchange interaction to act onbetween crystal grains. This increases noise resulting from the zigzagmagnetic domain wall in the transition region between recording bits.This is not desirable from the view point-of the recording/reproducingcharacteristics of the medium. Thus, it is necessary to non-magnetizethe grain boundary so as to eliminate exchange interaction betweencrystal grains.

As mentioned above, there is a contradiction between the magneticcharacteristics required of crystal grains proper and the magneticcharacteristics required of the grain boundary.

In order to realize a medium which satisfies both characteristics, thefilm is formed from a Co—Cr alloy magnetic material by a process whichemploys a higher sputtering energy than the related art sputteringprocess and the resulting film undergoes heat treatment (as mentionedlater in detail). This process permits the added elements to diffuse(for segregation) from the inside of magnetic crystal grains to thecrystal grain boundary. In this way it is possible to realize themagnetic recording medium which meets the above-mentioned requirements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing the structure of the magneticrecording medium pertaining to the present invention for comparison;

FIG. 2 is a schematic diagram showing the structure of the magneticrecording medium of the related art technology;

FIG. 3 is a graphical representation illustrating the magnetic torquewhich occurs when the magnetic field is applied in the direction of 45degrees;

FIG. 4 is a graphical representation illustrating the change withtemperature of the magnetic anisotropy constant measured;

FIG. 5 is a graphical representation illustrating the temperaturedependence on the ratio of magnetic anisotropy energy to thermalvibration energy; and

FIG. 6 is a schematic diagram showing the magnetic storage device withthe magnetic recording medium of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described in more detail with reference tothe following examples and drawings. The identical reference numerals inFIGS. 1 and 2 denote those parts which have the same characteristics andperformance.

FIG. 1 shows the structure of a magnetic recording medium forlongitudinal recording. The magnetic recording medium is composed offollowing layers.

1: a quartz substrate of the medium (for 3.5-inch disk).

2: an underlying film formed on said quartz substrate 1, which isintended to control the orientation of the magnetic film.

5: an underlying film formed on said underlying film 2, which isintended for lattice matching.

3: a magnetic recording film (which is a thin film of cobalt alloy)formed on said underlying film 5.

4: a protective film of carbon formed on said magnetic recording film 3.The underlying film 5 for lattice matching is special to this example.It is provided for lattice matching with the magnetic film 3.

A cleaned quartz substrate 1 was placed in a sputtering apparatus. Withthe apparatus evacuated to a degree of vacuum 1×10⁻⁸ Torr or lower, thesubstrate 1 was heated to 300° C. and then allowed to stand at aconstant temperature for 1 hour. On this substrate 1 were formedsequentially a Cr film (10 nm thick) as the underlying film 2 fororientation control and a Cr-(15 at. %)Ti film (20 nm thick) as theunderlying film 5 for lattice matching. The underlying films 2 and 5 (ordouble-layer structure) were formed at a rate of 2 nm/sec by the DCmagnetron sputtering, with the Ar gas pressure maintained at 3 mTorr.

On the multilayer underlying films 2 and 5 was formed the magneticrecording film (12 nm thick) having the average composition of Co-(15at. %)Cr-(12 at. %)Pt-(3 at. %)Ta by ECR (Electron Cyclotron Resonance)sputtering which generates more energy than DC magnetron sputtering. Thetargets (excluding the C target) have a purity of 99.9%. The alloy thinfilm was formed from an alloy target having a composition correspondingto that of the film. The composition of each film mentioned above is anaverage composition of single film determined by ICPS (InductivelyCoupled Plasma Spectroscopy).

The thin film sample prepared as mentioned above was subsequentlytransferred again to the heating stage of the sputtering apparatus. Itunderwent heat treatment in a vacuum in the following way. With thechamber evacuated to 5×10⁻⁹ Torr, the sample was heated at a constanttemperature of 450° C. for 15 minutes, followed by cooling to roomtemperature. Finally, on the outermost surface was formed a protectivefilm of carbon (15 nm thick) at room temperature.

The recording medium prepared as mentioned above is designated as sampleA. For the purpose of comparison, a recording medium of the related artstructure was prepared, which is designated as sample B.

FIG. 2 shows the structure of sample B. In sample B, the magnetic film 3is composed of Co-(16 at. %)Cr-(4 at. %)Ta. This magnetic film has athickness of 12 nm, which is the same as that of sample A, for easycomparison of their characteristics. Sample B was prepared in thefollowing way.

A cleaned substrate 1 of NiP plated aluminum alloy for magnetic disk wasplaced in a sputtering apparatus. With the apparatus evacuated to avacuum lower than 1×10⁻⁸ Torr, the substrate 1 was heated to 270° C. andthen allowed to stand at a constant temperature for 1 hour. On thissubstrate 1 were formed sequentially a Cr film (50 nm thick) as theunderlying film 2 for orientation control, a Co—Cr—Ta magnetic film (12nm thick) as the magnetic recording film 3, and a protective film 4 ofcarbon. Sputtering was carried out at a rate of 2 nm/sec by the DCmagnetron sputtering, with the Ar gas pressure maintained at 3 mTorr.The targets (excluding the C target) have a purity of 99.9%. Themagnetic film was formed from an alloy target.

Thus prepared samples A and B were tested for recording/reproducingcharacteristics. Small pieces were cut out of them and they wereexamined for film structure and magnetic properties. The film structurewas investigated by θ-2θ x-ray diffractometry. In sample A, the Cr—Tiunderlying film gave reflection due to the (200) plane and (110) plane,with the latter having an intensity which is about one-tenth of that ofthe former. Also, in sample A, the magnetic film gave main reflectiondue to the (110) plane and reflection due to the (101) plane (whoseintensity is about {fraction (1/15)} of that of main reflection).

In sample B of the related art structure, the Cr underlying film gavereflection due to the (200) plane and the Co—Cr—Ta magnetic film gavereflection due to the (11.0) plane of hexagonal close-packed structure.

The results of sample A suggest that the crystal grains of the Cr—Tiunderlying film are mostly oriented in the (100) direction and hence themagnetic film, which has epitaxially grown on this underlying film, isoriented in the (110) direction, and that the crystal grains of theCr—Ti underlying film are partly oriented in the (110) direction andhence the magnetic film has grown, with orientation in the (101)direction, according to the part oriented so. The diffraction plane ofthe magnetic film is indicated by four indices, with the third termomitted. Incidentally, the Cr underlying film gave no distinct peak ofx-ray diffraction.

Both samples were examined for the microstructure of crystals byobserving the plain TEM image of the magnetic film with an electronmicroscope. In sample A, the average crystal particle diameter was about10.5 nm. In sample B of the related art structure, the average crystalparticle diameter was about 13 nm, with a broader particle diameterdistribution. The particle diameter distribution of sample A is about20% smaller than that of sample B. The investigation of the sectionalstructure of crystal grains revealed that the crystal lattice continuesfrom the Cr—Ti film for orientation control to the magnetic film. Thissuggests that good crystals were obtained by epitaxial growth withlattice matching.

Both samples were also examined for the composition distribution ofindividual crystal grains. The composition at an arbitrary point on themagnetic film was analyzed by using an analytical electron microscopeequipped with an EDX analyzer (Energy Dispersive method in X-raySpectroscopy) having a space resolving power of 2 nm. It was found that,in sample A, the content of Cr at the crystal grain boundary is as highas 25 at. %, whereas the content of Cr in the crystal grain is about 8at. % on average. In other words, it was found that the Cr content incrystal grains is only one half of that of the average film composition.It was proved that heat treatment after film forming causes the addedelement to diffuse (for segregation) from the inside of magnetic crystalgrains to the crystal grain boundary. Thus, it was possible to obtainthe structure which contributes to reduction of noise due to medium, asmentioned above.

Then, the magnetic anisotropy energy of the magnetic film was examined.The measuring method is explained below. Since the medium is apolycrystalline thin film in which crystals are oriented and dispersed,it is usually difficult to obtain the magnetic anisotropy energy of themedium material. One way to get around this difficulty is to determinethe magnetic anisotropy constant by measuring the magnetic torque of asingle-crystal thin-film sample prepared from the magnetic material usedfor the medium. On the other hand, there is a prevailing means to obtainthe magnetic anisotropy of the medium sample as such. That is, theanisotropic magnetic field of the medium is indirectly estimated bymeasuring the dependence of the rotational hysteresis loss of the mediumon the applied magnetic field. Unfortunately, the anisotropic magneticfield obtained in this way is vulnerable to the magnetic mutual actionbetween magnetic crystal grains and the orientation and dispersion ofcrystals.

In order to remedy this drawback, there has been proposed a method ofextrapolating the magnetic anisotropy energy of a medium from themagnitude of magnetic torque which is measured when a magnetic field isapplied in the direction of 45 degrees with respect to the film surfaceof the medium. (See IEEE Transaction on Magnetics, vol. 32, No. 5(September 1997), pp. 4902-4904, as the fourth US literature.) Thismethod permits higher precision of measurements than the related artmethods because it treats statistically the effect of crystalorientation and dispersion. The method of measuring the magnetic torqueat 45 degrees is best at present for measurements of the magneticanisotropy of an actual medium. Hence, it was adopted for measurementsof samples in this example.

Samples A and B were tested for the temperature dependence of magneticanisotropy in the range of 300 K to 375 K in the following manner byusing a magnetic torque meter. A magnetic torque meter was attached tosample A such that a magnetic field was applied in the direction of 45degrees with respect to the film surface of the sample. With the sampleheated at T=300 K, a magnetic field H_(ex) was applied within a range of9 kOe to 13 kOe, at a step of 0.5 kOe to measure the magnetic torqueL_(45deg)(H_(ex)) detected. FIG. 3 shows the relation between[L_(45deg)(H_(ex))/H_(ex)]² and L_(45deg)(H_(ex)). It is noted that therelation is approximately linear. This linear relation gives an equation(1) below, in which L_(o) stands for a constant for the intersection onthe L_(45deg) axis and M_(s) stands for the average saturationmagnetization of the film.

L _(45deg)(H _(ex))=L _(o)−[2L _(o) /M _(s)]² [L _(45deg)(H _(ex))/H_(ex)]²  (1)

The constant L_(o) and the slope [2L_(o)/M_(s)]² were calculated by theleast-squares method and the values of L_(o) and M_(s) were obtainedfrom them.

In the case of sample A, L_(o)=4.6×10⁶ erg/cc and M_(s)=540 emu/cc atT=300 K, and hence the inequality L_(o)≧2πM_(s) ² holds. Therefore,K_(u)=2[L_(o)−2πM_(s) ²]=5.6×10⁶ erg/cc, which satisfies K_(u)(T=300K)>3×10⁶ erg/cc. The constants of magnetic anisotropy at differenttemperatures (T=325 K, 350 K, and 375 K) were obtained in the same wayas mentioned above. FIG. 4 shows the temperature dependence of K_(u)thus obtained. In the case of sample A, the value of K_(u) decreased byonly about 10% when the temperature rose from 300 K to 350 K. Bycontrast, in the case of sample B, the value of K_(u) at 300 K was1.2×10⁶ erg/cc and the value of K_(u) greatly decreased by about 20%when the temperature rose from 300 K to 350 K.

For investigations on the stability of the state of magnetization, thechange of K_(u)V/k_(B)T with temperature was observed. The results areshown in FIG. 5. In the case of sample A, K_(u)V/k_(B)T=140 at T=300 Kand K_(u)V/k_(B)T=112 at T=350 K. In other words, the value ofK_(u)V/k_(B)T decreased by about 20% when the temperature rose by 50 K.This amount of change with temperature coincides with the sum of theamount of decrease proportional to 1/T and the amount of decrease ofK_(u) itself occurred due to temperature rise. In case of sample B,K_(u)V/k_(B)T=85 at T=300 K and K_(u)V/k_(B)T=58 at T=350 K. In otherwords, the value of K_(u)V/K_(B)T greatly decreased by 30% or more whenthe temperature rose by 50 K.

FIG. 6 shows an embodiment of the magnetic storage device (magneticrecording/reproducing device) which runs the above-mentioned magneticrecording medium. The magnetic storage device is constructed of thefollowing components.

71: a magnetic recording medium mentioned above.

72: a magnetic head (opposing to the magnetic recording medium) whichpicks up magnetic information as electrical signals.

73: a suspension to hold the heads 72.

74: an actuator.

75: a voice coil motor to drive the actuator 74.

77: a head positioning circuit.

Electrical signals from the magnetic head 72 are introduced to arecording/reproducing circuit 76. Electrical signals to and from thismagnetic storage device pass through an interface circuit 78. The medium71 is driven by a motor 79.

For investigations on the stability of the state of magnetization inactual recording, the magnetic storage device shown in FIG. 6 was run toperform recording in a linear density of 200 kFCI. The reproducingoutput was measured immediately after recording and about 100 hoursafter recording. Recording was carried out by using a thin-film headwith a track width of 2.5 μm and a gap length of 0.3 μm. Reproducing wascarried out by using a magneto-resistive head with a track width of 2 μmand a shield distance of 0.2 μm. During recording and reproducing, thehead was held 0.06 μm above the surface of the protective film of themedium. The speed of the slider relative to the substrate was kept at 11m/s.

Recording with a linear density of 200 kFCI was carried out at a normaltemperature of T=300 K. Immediately after recording, reproducing wascarried out. The recording medium was allowed to stand at 300 K for 100hours. Then, reproducing was carried out again. In the case of sample A,the reproducing output after standing for 100 hours was about 99% of thereproducing signal output measured immediately after recording. Inconsideration of experimental errors, it can be said that there was verylittle decrease in reproducing output even after standing for 100 hours.

Recording with a linear density of 200 kFCI was carried out at a raisedtemperature of T=350 K. Reproducing was carried out immediately afterrecording and 100 hours after recording. The reproducing output afterstanding for 100 hours decreased by only 2% from that measuredimmediately after recording. In other words, the state of recordingmagnetization remained sufficiently stable.

By contrast, in the case of sample B, the reproducing output 100 hoursafter the recording showed about 5% and about 12% decrease, lower thanthat immediately after recording at T=300 K and T=350 K, respectively.This result indicates that the magnetic A storage device with sample Bdoes not keep recording bits stable over a long period of time.

It has been demonstrated in the foregoing that the present inventionprovides a magnetic recording medium which changes in magneticanisotropy energy only a little with temperature within the operatingtemperature range of the apparatus, and which has a low noise level andis suitable for high-density recording, keeping the recording andreproducing characteristics stable over a long period of time. Thepresent invention also provides a magnetic storage device with highreliability and large capacity which is based on said magnetic recordingmedium.

What is claimed is:
 1. A magnetic recording medium including a magneticrecording film, wherein the magnetic recording film is characterized inthat its magnetic anisotropy energy constant K_(u) at an absolutetemperature T of 300 K is K_(u) (T=300 K)≧3.6×10⁶ erg/cc and the averageparticle diameter (d) of magnetic crystal grains is 5 nm<d<12 nm, whered is defined as the diameter of a circle having the same area as that ofa magnetic crystal grain in the direction of the plane of the film, andthe magnetic recording film satisfies the following relationship: K_(u)(T=350 K)/K _(u)(T=300 K)≧0.85 where K_(u) (T=350 K) is the magneticanisotropy energy constant at an absolute temperature T of 350 K andK_(u) (T=300 K) is the magnetic anisotropy energy constant at anabsolute temperature T of 300 K.
 2. A magnetic recording mediumincluding a magnetic recording film, wherein the magnetic recording filmis characterized in that its magnetic anisotropy energy constant K_(u)at an absolute temperature T of 300 K is K_(u) (T=300 K)≧3.6×10⁶ erg/ccand the average particle diameter (d) of magnetic crystal grains is 5nm<d<12 nm, where d is defined as the diameter of a circle having thesame area as that of a magnetic crystal grain in the direction of theplane of the film, and the magnetic recording film satisfies thefollowing relationship: [K _(u) V/K _(B) t](T=350 K)/[K_(u) V/K _(B)t](T=300 K)≧0.73 where V is the average volume of magnetic crystalgrains, K_(u) is the magnetic antisotropy constant of magnetic crystalgrains, K_(B) is the Boltzmann constant, and T is the absolutetemperature, with the product of K_(B) and T representing the energy ofthermal vibration.
 3. A magnetic recording medium according to claim 1,wherein the magnetic film satisfies the following relationship: [K _(u)V/K _(B) t](T=350 K)>85 where V is the average volume of magneticcrystal grains, K_(u) is the magnetic antisotropy constant of magneticcrystal grains, K_(B) is the Boltzmann constant, and T is the absolutetemperature, with the product of K_(B) and T representing the energy ofthermal vibration.
 4. A magnetic recording medium according to claim 2,wherein the magnetic film satisfies the following relationship: [K _(u)V/K _(B) t](T=350 K)>85 where V is the average volume of magneticcrystal grains, K_(u) is the magnetic antisotropy constant of magneticcrystal grains, K_(B) is the Boltzmann constant, and T is the absolutetemperature, with the product of K_(B) and T representing the energy ofthermal vibration.
 5. A magnetic storage device with a magneticrecording medium including a magnetic recording film, wherein themagnetic recording film is characterized in that its magnetic anisotropyenergy constant K_(u) at an absolute temperature T of 300 K is K_(u)(T=300 K)≧3.6×10⁶ erg/cc and the average particle diameter (d) ofmagnetic crystal grains is 5 nm<d<12 nm, where d is defined as thediameter of a circle having the same area as that of a magnetic crystalgrain in the direction of the plane of the film, and the magneticrecording film satisfies the following relationship: K _(u)(T=350 K)/K_(u)(T=300 K)≧0.85, where K_(u) (T=350 K) is the magnetic anisotropyenergy constant at an absolute temperature T of 350 K and K_(u) (T=300K) is the magnetic anisotropy energy constant at an absolute temperatureT of 300 K.
 6. A magnetic storage device with a magnetic recordingmedium including a magnetic recording film, wherein the magneticrecording film is characterized in that its magnetic anisotropy energyconstant K_(u) at an absolute temperature T of 300 K is K_(u) (T=300K)≧3.6×10⁶ erg/cc and the average particle diameter (d) of magneticcrystal grains is 5 nm<d<12 nm, where d is defined as the diameter of acircle having the same area as that of a magnetic crystal grain in thedirection of the plane of the film, and the magnetic recording filmsatisfies the following relationship: [K _(u) V/K _(B) t](T=350 K)/[K_(u) V/K _(B) t](T=300 K)≧0.73, where V is the average volume ofmagnetic crystal grains, K_(u) is the magnetic antisotropy constant ofmagnetic crystal grains, K_(B) is the Boltzmann constant, and T is theabsolute temperature, with the product of K_(B) and T representing theenergy of thermal vibration.